The invention relates to an operational amplifier (OA) with reduced offset voltage, particularly for electronic circuits where fast and precise signal amplification is required. Among these applications are A/D and D/A converters, S/H amplifiers as well as measuring instruments. The OA may also be used in all applications where up until now fast or precision OA's have been employed.
The commonly used fast OA's are very inaccurate. Their offset voltages and input currents can be very high. Another disadvantage is the temperature dependence of the various values. The accuracy of the expensive hybrid OA's is hardly better. The precision OA's are extraordinarily slow and are therefore used only for very specific and limited applications. The offset voltage and its temperature dependence are extremely small, input currents are negligible.
Almost always the offset voltage of an OA may be adjusted externally by using a potentiometer. However, this does not improve either the temperature dependence of the voltage of the rest of the values. The long-term stability of the entire circuit is adversely affected by the use of the mechanical component.
The invention is intended to provide an OA with all the advantages of a fast and precision OA. According to the invention, this problem is solved by the fact that a circuit is designed with at least four resistors and two OA's. According to a practical embodiment, a fast OA is employed. It is recommended, however not necessary, to employ a precision OA as a complement.
The advantages offered by the invention are mainly that the advantages of both groups of the OA's are combined in one device, something not possible up until now. Moreover, the possibilities created by the employment of the two OA's come into play. These are in particular the adjustment of the input currents and their temperature dependence of the fast operational amplifier.
In most cases, merely ratios of values of the individual resistors become necessary. The design of the precision OA may be simplified considerably, since the direction of its output current can be determined. A push-pull output stage is not necessary.